Data from: Dissecting factors behind temporal trends in the timing of breeding in two songbird species – evolutionary change or phenotypic plasticity?

Warming climate has led to significant phenological advances in many plant
and animal populations. Whether these advances represent evolutionary
responses or phenotypic plasticity remain typically unknown. Using 53
years long time-series of individually marked Great (Parus major) and
Willow Tits (Poecile montanus), we investigated whether the significant
breeding time advances in these species could be explained to result from
evolutionary responses, phenotypic plasticity, or both. In the case of
both species, we did not find any evidence for changes in breeding values
for timing of breeding, suggesting that the observed changes do not have a
genetic, and hence, evolutionary basis. In contrast, we found that
annually fluctuating environmental effects explained most of the variation
in first egg lay dates, suggesting that advances in breeding time were
attributable to phenotypic plasticity. We further inferred that phenotypic
plasticity in response to spring temperatures can fully explain the
observed advancement of Great Tit phenology over time, whereas Willow Tits
have advanced their phenology much beyond what would be expected from
phenotypic plasticity in response to spring temperatures. The latter
observation suggests that some other yet unidentified environmental
factor, uncorrelated with spring temperatures, likely explains about half
of the advancement in their breeding time.